Telematics units within mobile vehicles provide subscribers with connectivity to a telematics service provider (TSP). The TSP provides subscribers with an array of services ranging from emergency call handling and stolen vehicle recovery to diagnostics monitoring, global navigation system aided position identification, map services, and turn-by-turn navigation assistance. Telematics units are often provisioned and activated at a point of sale when a subscriber purchases a telematics-equipped vehicle. Once provisioned and activate, telematics units can be utilized by a subscriber to obtain telematics services, such as those described herein, from the TSP.
One service provided by a TSP is a Global Navigation Satellite System (GNSS) based navigation service. GNSS is a generic term referring to any of multiple satellite constellation-based global positioning networks including, for example, the Global Positioning System (GPS) constellation. Another GNSS constellation network is the Global Navigation Satellite System (GLONASS). A particular example of a GNSS based navigation service is the provision of turn-by-turn (TBT) directions from a current location of a vehicle or an alternative start location to a specified destination.
However, under certain conditions, GNSS signals received by the telematics unit in a vehicle are too weak to provide meaningfully accurate information regarding the position of the vehicle. For example, the telematics unit in a vehicle traveling in a parking garage, tunnel, or other underground or covered structure may be unable to receive GNSS signals. Under such conditions, the driver of the vehicle is unable to rely on the GNSS navigation services provided by the TSP. In order to provide navigation services under conditions where GNSS signals are too weak, the telematics unit must rely on a Dead Reckoning (DR) system. However, DR systems are not able to provide position information with the level of accuracy provided by a GNSS.